Various methods have been used to arrange papers together for storage and reading. Loose sheets of paper can be permanently bound by gluing, sewing, stapling, and the like. Papers can also be held together with readily removable fasteners and releasable binders, for example. Such fasteners commonly have an enlarged head with a stem that is inserted through a punched hole with the stem end then bent radially outward at the back of that hole. In addition, binders, such as two-ring and three-ring binders, have spring biased rings which hold loose sheets of paper together. Such releasable binders and removable fasteners permit the easy binding of loose sheets of paper, yet permit the ready removal of the papers for copying, for instance.
Among the more important requirements in making a hole for paper binding purposes is that the hole be uniform, neat and properly aligned with other holes in papers to be bound. That is, the holes should look the same after every punch, with the holes made so that their edges are precise, and not jagged.
Ordinarily a hole will be made through many sheets of paper at one time. As many as twenty to thirty sheets may be simultaneously placed in a manually-driven punching device, for example, such as a three-hole punch operated by hand force.
Various punch pin configurations have been used to make sharp and uniform holes. These pins are typically cylindrical in shape, with a base having a circular cross section. The variations in the pins are generally found in the shape of the punch pin base.
Originally, such punch pins had a flat circular cutting edge. However, it became apparent that these pins required large amounts of force to perforate the paper due to the large surface area of the paper being engaged at one time by the cutting surface of the pin. In addition, because the entire surface area of the base cut the paper all at once, higher shear forces were applied to the edges of the hole being created. This caused the paper to be "pulled" around the edges into the hole, resulting in a dull hole edge.
Another punch pin cutting edge configuration is shown in U.S. Pat. No. 3,714,857. The punch pin is cylindrical with a generally circular cross section. The cutting edge of the punch pin has a parabolic-shape when viewed in section.
The punch pin pierces the paper along a smaller cross section of the edge at the base of the cylinder. The paper furthermore contacts a variably changing cutting edge, so that the entire edge of the base never comes into contact with the paper at once. This reduces the force necessary to cut the paper because of the reduced surface contact between the pin and the paper. Conversely, using the same amount of force to pierce the paper results in a greater pressure on the paper being cut because of the lower surface contact. This allows the user to cut more sheets of paper with the same force. This higher pressure over a smaller area also results in a sharper cut, because the "pull" on the edge of the hole is reduced.
Other punch pin configurations have also focused upon a reduction in the force necessary to make the paper hole. Such configurations have included highly sloped piercing points as well as rippled or star-shaped patterns on the cutting edge. There is thus a desire in the industry to develop a cutting edge configuration for a punch pin which creates a sharp, clean cut with the least amount of force possible.